This project concerns magnetic resonance imaging (MRI) as a probe of breast-tissue capillary-bed topology. Previous breast MRI studies based on T1 or T2 weighting, or blood flow have failed to reduce the number of surgical biopsies or affected patient treatment. Whereas capillary density increases non-specifically in breast cancers, the size, shape, and permeability of the capillaries change more specifically. Contrast agents that increase the magnetic susceptibility in the intravascular spaces are now available. When the magnetic susceptibility in the intra- and extravascular spaces is different, field perturbations are setup to which MRI is sensitive Formulas for modeling the perturbations caused by such agents have been corrected. These, coupled with "Monte Carlo" simulations of water proton diffusing randomly through the perturbed field, yield results which agree quantitatively with measurements. In this project, we will model different types of capillary beds as col lection of finite cylinders and perform Monte Carlo simulations of water proton diffusion while changing 1) the magnetic susceptibility, and 2) the MRI pulse sequence parameters. We will validate the model using corrosion casts of capillary beds from murine breast tumors.